1. Field of the Invention
This invention relates to a vital apparatus for receiving signals transmitted over railroad rails. It is more particularly concerned with vital receivers having low impedance for such purpose.
2. Description of Prior Art
Communication over railroad rails using electrical signals had been accomplished using direct current, coded direct current or alternating current. Receivers to detect or decode such signals may be located on locomotives, transit vehicles, or wayside, wherever it is desired to interface with such rail signals. Steady state direct current permits transmission of only one piece of information; coded DC or AC can accommodate many specific communications. Where communication over distances as great as several miles is desired, the high and variable losses in rail transmission lines requires signals of very low frequencies such as 2 to 10 Hertz. It is fairly often desired to use track receivers in track circuits where the loss of detection of such signals is an indication that either the track is occupied or that the rail is broken. Because of variations and transmission losses in rail-to-rail impedance caused by ballast conditions, it is necessary to use a receiving device of extremely low impedance, usually under 1 ohm. It is essential for safety considerations that detection be free from ambiguity; therefore, the input impedance of the receiver must not, as the result of any component failure, have an increased input impedance without a corresponding loss in sensitivity. A conventional receiver with input shunted by a resistor would be unsafe if the resistor or its wiring should open because its input impedance would increase and the receiver sensitivity would also increase.